There are numerous patents dealing with enhancing the degradability of conventional non-biodegradable polymers such as polyolefins by use of additive systems. These additive systems are quite frequently designed to enhance the polymers degradability in a specific type of environment and over a specific length of time. For example, U.S. Pat. No. 3,840,512 (Brackman) exemplifies prodegradant systems comprising ferric stearate with various free fatty acids, both saturated and unsaturated. Manganese stearate is also exemplified in a system with stearic acid. Brackman states that thermoplastic films (e.g., polyolefin films) formed with these prodegradant systems will embrittle when exposed to artificially UV-activated irradiation at times ranging from 2 to 35 days. It is specifically stated that the nature of the hydrocarbon group on the fatty acid does not have a large influence on the rate of UV degradation. Brackman does not address the issue of degradability in other environments, such as in a compost environment. A patent dealing with a similar prodegradant system, U.S. Pat. No. 4,067,836 (Potts et al.), discloses adding a transition metal salt, an auto-oxidative susceptible additive, and an anti-oxidant to polyethylene. The only exemplified auto-oxidative susceptible additives were polypropylene and polyethylene oxide (which did not work as acceptably as polypropylene). The degradation of the samples was tested by exposure to an artificial solar light spectral distribution. The degradability characteristics of these prodegradant additives were never demonstrated in other environments such as a compost environment. Generally, additive systems as described above, designed to make a polymer degrade when exposed to environmental radiation, have proved of doubtful practical utility. Only a relatively small portion of the waste stream is ever exposed to sunlight, even for short periods of time.
In U.S. Pat. No. 3,921,333 (Clendinning, et al.) it is proposed to make the composition of Potts, et al., discussed above, degradable in a soil type environment by adding a biodegradable polymer such as poly(caprolactone). The invention described is allegedly useful for materials such as transplanting containers, mulch film and the like. Again, only a small portion of the plastic in the waste stream is ever used in such environments and as such the compositions described are of limited applicability based on their limited intended use.
U.S. Pat. No. 4,038,228 (Taylor, et al.) describes placing a transition metal salt of an unsaturated organic acid or ester into a polymer film (e.g.,, polyethylene or polypropylene) to enhance its degradability in the absence of sunlight. The transition metal salts discussed are identical to many of those exemplified in the above Clendinning et al. and Potts et al. patents; however, they are exemplified at extremely high concentrations. The exemplified film degrades to an embrittled condition within three days at room temperature. Such a film is of doubtful utility as it would likely degrade before use and the exemplified high concentrations of cobalt used would create an extremely costly and toxic material.
A more recent patent, U.S. Pat. No. 4,931,488 (Chiquet), describes a polymer (e.g., polyethylene) composition which allegedly will degrade when exposed to heat, ultraviolet radiation, sunlight, or under composting conditions. The prodegradant system broadly described consists of a biodegradable substance such as starch, an iron compound and a fatty acid or fatty acid ester, optionally with copper stearate. The exemplified films, however, are limited to polyethylene blended with ferric stearate and soya oil, with a minor proportion of cupric stearate in certain examples. Although it is alleged that these compositions are tested under composting conditions, the conditions are not actually set forth and the reported films do not appear to degrade for up to twenty weeks, a situation which would be unacceptable in most commercial composting situations where peak temperatures are reached for only approximately two weeks.
As can be seen the art continues to seek to improve the degradability of conventional plastic films in various environments by use of additive prodegradant systems. These systems have been designed to provide degradability properties in a wide variety of environmental conditions. Systems that have been found to work in one list of conditions do not necessarily work under a separate set of conditions which can vary from a dry sunlit exposure to the wet, dark, and relatively infertile conditions of a composter.
Several patents have also dealt with the degradability of multilayered structures formed from various polymers. For example, U.S. Pat. No. 3,647,111 (Stager et al.) discloses a biodegradable container formed from a core layer of an organic filler material, such as peat, and a phenolic resin impregnated with a metallic salt of a fatty acid. This core layer is laminated with an outer protective coating, such as a water-soluble paint, and an inner protective coating, such as a thin plastic liner, a metal flash, or a very thin layer of wax. Allegedly, the extremely thin inner layer, which performs no significant structural function, will break down under normal atmospheric conditions.
U.S. Pat. No. 5,009,648 (Aronoff et al.) discloses biodegradable films comprising starch blended with a polymeric material, such as ethylene vinyl acetate, as well as ostomy pouches formed from such films. Supposedly, these films and pouches will degrade when deposited in a landfill or compost heap after transit through a municipal sewage system and collection at a sewage treatment plant.
Furthermore, U.S. Pat. No. 5,108,807 (Tucker) discloses degradable multilayer thermoplastic articles comprised of a water-soluble and/or biodegradable core layer surrounded by two opposing layers of thermoplastic polymers containing an effective amount of a prodegradant, such that these layers will degrade by photo, thermal, or chemical means. Such articles are disclosed to be useful in the construction of bags formed from multilayered films.
In a different aspect, considerable research has been conducted regarding the formation of radiation resistant polymeric compositions and structures. In this regard, attempts have been made to overcome degradation problems associated with crystalline polypropylene. For example, mesomorphous polypropylene, as described in U.S. Pat. No. 4,931,230, and articles manufactured from mesomorphous polypropylene, such as described in U.S. Pat. No. 4,950,549, provide resistance to sterilizing irradiation. In addition, polymer blends of mesomorphous polypropylene and a polymer compatible with such polypropylene, as described in U.S. Pat. No. 5,140,073, have been developed. By controlling the method of preparing mesomorphous polypropylene, through the quenching of such polypropylene after hot-melt extrusion, the material or articles formed therefrom substantially maintain their structural integrity after exposure to ionizing radiation at dosages sufficient to degrade crystalline polypropylene.
Applicants have found a composition which will rapidly degrade under conditions of a typical commercial composting unit yet provide an article such as a film which is functional under normal use conditions. A typical composting unit generally is exposed to peak temperatures of greater than 60.degree. C. for periods of approximately two weeks or less. During that period, the organic matter in the composter is generally exposed to an extremely high humidity, generally close to one hundred percent. These humidity conditions are generally favorable for biological attack, however, they are generally inhospitable to oxidative type degradations where transition metal salts are typically employed.
In addition, Applicants have also discovered various multilayer polyolefin structures that will degrade under a variety of conditions, including the commercial composting conditions described above. Surprisingly, such degradation will occur when one or more of the polyolefin layers is lacking in a prodegradant system. Also, such degradation can surprisingly be accomplished with compositions, articles, and structures formed from radiation resistant, mesomorphous polypropylene.